A gas turbine power plant such as a combined cycle or cogeneration power plant generally includes a gas turbine having a compressor, a combustor, a turbine, a heat recovery steam generator (HRSG) that is downstream from the turbine and a steam turbine in operably connected to the HRSG. During operation, air enters the compressor via an inlet system and is progressively compressed as it is routed towards a compressor discharge casing prior to entering the combustor. A portion of the compressed air is mixed with a fuel and burned within a combustion chamber defined within the combustor, thereby generating high temperature and high pressure combustion gas.
The combustion gas is routed along a hot gas path from the combustor through the turbine where they progressively expand as they flow across alternating stages of stationary vanes and rotatable turbine blades which are coupled to a rotor shaft. Kinetic energy is transferred from the combustion gas to the turbine blades thus causing the rotor shaft to rotate. The rotational energy of the rotor shaft may be converted to electrical energy via a generator. The combustion gas exits via an outlet of the turbine as exhaust gas and the exhaust gas is routed to the HRSG. Thermal energy from the exhaust gas may be transferred to water flowing through one or more heat exchangers of the HRSG, thereby producing superheated steam. The superheated steam may then be routed into the steam turbine to generate additional electricity and/or may be routed to support various secondary operations at the power plant or cogeneration site, thus enhancing overall power plant or cogeneration capability.
Output of the gas turbine may be restricted at certain ambient and load conditions in order to avoid mechanical/vibrational limitations of turbine rotor blades positioned at or proximate to the turbine outlet. This output restriction is related to the axial exit velocity of the exhaust gas as it exits the turbine outlet. Once a maximum axial exit velocity or Mach number is reached, the output of the gas turbine is reduced or derated by controls methodology in order to not exceed the allowable mechanical limits. As a result, generator/power output and/or thermal energy output for combined or cogeneration cycle operation is reduced.